The ability of subpopulations of murine spleen cells to stimulate a mixed lymphocyte response (MLR) was studied. It was found that T cells (nylon nonadherent spleen cells) and B cells (G10 passed and treated with rabbit anti-mouse brain serum (RAMB) and complement (C)) were poor stimulators of an MLR. In contrast whole spleen cells or B cells plus adherent cells (RAMB plus C treated spleen cells) produced good stimulation. However, a non-T, radiation resistant splenic adherent cell (SAC) population was up to 20-50 times more efficient as a stimulator of an MLR on a per cell basis than an unseparated spleen population. These SAC were shown to express Ia (I-region associated) determinants encoded by genes in I-A and I-E/C. These results suggest that Ia plus SAC may be the predominant stimulating cells in spleen cell populations, and the preferential target for T cell recognition in cell interaction events.